Process and apparatus for removing pollutants from a combustion gas

Abstract: In addition to the conventional stages and parts of methods and plants of the post-combustion type in which SO₂ and NO X are removed mainly by means of the technique of energizing the combustion gas by means of electrons, at least one heat exchanger (7, 8) is comprised in the energizing zone (5, 6) with the purpose of condensing water on the walls of the heat exchanger (7, 8) so as to produce reactions in the heterogeneous and humid phase between NO₂, SO₂, H₂O, and NH₃ that increase the effectiveness of the NO X removal and comprises means (M) to inject into the plant the condensation gathered by said walls, so as to gather the salts contained in the same.

Description: The present invention concerns a method for the removal of pollutants from a combustion gas and a plant for carrying out the method, wherein, in a post-combustion type plant, SO 2 and NO X are removed with the gas energizing technique. According to the state of the art, with the above mentioned methods and plants the removal of SO 2 generally takes place through thermo-chemical reactions, even when there is no energy transferred to the gas, and through radical reactions, when energy is transferred to the gas. The radicals produced by the transferral of energy to the gas similarly provide for the removal of nitrogen oxides. Firstly, the interaction between the gas and electrons of suitable energy is caused, so as to transform the SO 2 and NO X , respectively into H 2 SO 4 and HNO 3 and, hence, an input of NH 3 , so as to convert these acids into ammonia salts and, lastly, a collection of said salts in electrostatic precipitators and/or in sleeve filters. Within the state of the art we refer especially to a method and plant, as described by the applicant in EP-A-0 532 841, in which the removal of pollutants takes place conventionally by means of: cooling of the combustion gas; passage of the gas in at least a first electrostatic precipitator, to gather most of the light ashes; gas energizing, to partly convert SO 2 into H 2 SO 4 and NO X into N 2 , NO 2 , and HNO 3 ; and addition of ammonia, to convert the latter acids into ammonia salts, to be gathered in at least a further electrostatic precipitator. The main characteristics of this process are the maintenance of the temperature at around 100°C, so as to reduce the thermo-chemical conversion of the SO 2 into acidic sulphides and sulphates and, subsequently, the injection of H 2 O 2 into the gas, to convert the residual SO 2 into H 2 SO 4 which reacts with NH 3 to produce ammonia salts. The inconvenience of this state of the art is that it provides a limited removal of NO X , because of the strong concentration of NO 2 deriving from the radical conversion of NO. This inconvenience is, formally, avoided by accepting an NH 3 content of various dozens of ppmv in the output gas from the plant and, concretely, by inserting a sleeve filter into the plant; however, this causes blockage of the filter and leads to high operating costs and to interruptions in the running of the plant for maintenance activities. It is also worth mentioning that DE-A-3 907 703 discloses a method and an apparatus, pertaining the same field of the present invention, wherein the energizing step is splitted into a plurality of steps, and a washing operation with a chemical solution is carried out between the steps. The invention obviates the above mentioned inconveniences by providing a method and a plant as delineated, respectively, in claims 1 and 2. The main advantages of the invention are the greater effectiveness of the degree of removal of nitrogen and sulphur oxides, the elimination of the conventional sleeve filter, the reduction in the consumption of hydrogen peroxide and the capture of ammonia salts, as gathered from the above mentioned condensation, through water evaporation. Moreover, by means of the injection of the condensate, the problem of managing the liquids containing ammonia salts is solved and at the same time the capturing effectiveness of the first electrostatic precipitator is increased, since the ammonia salts injected do reduce the resistivity of volatile ashes;yet more, the thermo-chemical reaction between SO 2 and NH 3 on the heat exchangers is improved and this leads to a reduction of the quantity of H 2 O 2 to be injected downstream the second heat exchanger. The invention will be described below in further detail by means of a preferred embodiment as illustrated in FIG. 1 that represents a general diagram of the method and plant. Figure 1 shows a burner 1 in which coal is burned; a heat exchanger 2 in which the combustion gas temperature drops to 140 - 150°C; a first electrostatic precipitator 3 to remove up to about 90% of light ashes from the combustion gas; an electrode energizing structure 5, 6 similar to an electrostatic precipitator, e.g. including a series of parallel plates (not shown) connected to earth and interweaved with a series of frames bearing wires 5a, 5b, 6a, 6b fed by a positive impulse voltage from 70 to 100 kV, with a repeat frequency adjustable within a range between 30 - 300 impulses per second, so as to transfer to the combustion gas sufficient energy to remove nitrogen oxides in the desired percentage; two heat exchangers 7, 8 suited to condense the water contained in fumes upon their walls; means M to inject said condensate gathered by heat exchangers 7, 8 at the inlet to said first electrostatic precipitator 3; an injection system 4 suited to adding ammonia to the combustion gas and possibly also the condensate gathered by said heat exchangers 7, 8; means 9, 10 to gather the condensate produced by said heat exchangers 7, 8; a pump 11 that injects said condensate into the plant; a heat exchanger 12 to raise the temperature of the combustion gas; means 13 suited to injecting H 2 O 2 held in a container 14 in liquid or vapour form to convert residual sulphur dioxide into sulphuric acid that, reacting with the residual ammonia, produces ammonia sulphate; an electrostatic precipitator 15 that captures ammonia salts, especially the ammonia sulphate formed following the removal of polluting oxides; a blowing-sucking fan 16 upstream the plant funnel, not shown.

Claims: A method for the removal of pollutants from a combustion gas from a burner (1) in which the following stages take place : cooling the combustion gas arriving from the burner in at least a first heat exchanger (2); removing of up to about 90% of the light ashes contained in the gas in at least a first electrostatic precipitator (3); converting the sulphur dioxide (SO 2 ) and nitrogen oxides (NO X ), respectively into sulphuric acid (H 2 SO 4 ), nitric acid (HNO 3 ), nitrogen dioxide (NO 2 ) and nitrogen (N 2 ), with the addition of ammonia (NH 3 ) and by means of an electron energizing structure (5, 6), such as an electrostatic precipitator including a set of parallel plates connected to earth and interweaved with a series of frames-bearing wires (5a, 5b, 6a, 6b) fed by a positive impulse voltage from 70 to 100 kV, with a repeat frequency adjustable within a range between 30-300 impulses per second, to transfer to the combustion gas sufficient energy to remove nitrogen oxides in the desired percentage, and to convert said two acids into ammonia salts to be collected in at least one second electrostatic precipitator (15), the method being characterized in that it also comprises the stage of condensing part of the water contained in the combustion gases on the walls of at least a second heat exchanger (7, 8) located in the zone in which the gas energizing takes place, to produce reactions in the heterogeneous and humid phase between NO 2 , SO 2 , H 2 O, NH 3 , and in that it also comprises the stage of injecting at the entry of said first electrostatic precipitator (3), or else upstream or downstream the energizing stage, the condensate formed on said walls, so as to allow the capture of the salts thanks to water evaporation. A plant for carrying out the method of claim 1 comprising a heat exchanger (2), to cool the combustion gas arriving from a burner (1), at least a first electrostatic precipitator (3), to remove up to about 90% of the light ashes contained in the combustion gas in transit, means (4) to inject the ammonia into the combustion gas, at least one electron energizing structure (5, 6), such as an electrostatic precipitator including a set of parallel plates connected to earth and interweaved with a series of frame-bearing wires (5a, 5b, 6a, 6b) fed by a positive impulse voltage from 70 to 100 kV, with a repeat frequency adjustable within a range between 30-300 impulses per second, to transfer to the combustion gas sufficient energy to remove nitrogen oxides in the desired percentage, a blowing-sucking fan (16) upstream the plant funnel characterized in that the plant also comprises at least one heat exchanger (7, 8) located in the zone in which the gas energizing takes place and means (9, 10) to gather and inject within the plant the condensate as gathered, so as to allow the evaporation of the water and the capture of the salts contained in the condensation.

Inventors: CIVITANO LUIGI, CIVITANO, LUIGI, CIVITANO, LUIGI, C/O ENEL S.P.A. CENTRO RICERA
Applicants: ENEL SPA, ENEL S.P.A.
Application year: 1994
Main classification: B01D53
IPC codes: B01D53